1. Field of the Invention
The present invention relates to a method of operating a fiber-optic acoustical sensor, to an apparatus for use in practicing the method, and to an in-line fiber-optic polarizer usable in such an acoustical sensor. More particularly, the present invention is in the field of method and apparatus for a fiber-optic acoustical sensor used as a hydrophone (i.e., as a microphone used in water to receive sound transmitted through the water), and having particular utility in under-sea seismic exploration.
The present invention also relates to an in-line fiber-optic polarizer. This in-line fiber-optic polarizer is usable in an acoustical sensor embodying the invention.
2. Related Technology
Conventional electrical hydrophones are well known. In seismic exploration, these hydrophones are conventionally employed in static arrays of plural acoustical transducers which are placed on or beneath the sea floor, or in towed arrays (i.e., towed in sea water behind a transport ship or boat). The portion of the acoustical sensor immersed in sea water is generally referred to as the xe2x80x9cwetxe2x80x9d portion, while the portion on shore or aboard the transport vessel is the xe2x80x9cdryxe2x80x9d portion. The dry portion of the sensor may include signal analyzers, recorders, and display devices, for example. Connecting the wet and dry portions of the acoustical sensor is an elongate cable or cables extending between the acoustical transducers and the dry portion of the sensor. In some cases, the acoustical transducers are simply spaced out along the length of the connecting cable in a linear array located in the distal portion of the cable. This linear array configuration of acoustical transducers is generally used for towed arrays.
In the use of hydrophone arrays for seismic exploration, acoustical energy is provided in the water (by a sounding device or explosion, for example). Sound waves from the energy source travel through the water and penetrate into the earth at the sea floor. The sound waves are reflected from geological structures beneath the sea floor (i.e., from oil shale formations, for example), travel back into the sea water above, and are sensed by the acoustical transducers of the hydrophone array. These acoustic transducers (or hydrophones) thus provide signals indicative of the sound energy recovered from the reflections sensed at particular locations in the static or towed array. In this way, the undersea geological structures can be acoustically detected, and with the collection of sufficient data, can be acoustically xe2x80x9cimagedxe2x80x9d.
Because the recovery of sound energy is required at a multitude of spaced apart locations in order to acoustically image undersea geological structures, conventional hydrophone arrays include a multitude of acoustical transducers. Also, because of the large number of transducers in conventional electrical hydrophone arrays, the conventional electrical arrays have had to include a great number of electrical wires and active electrical circuits in the array itself as well as in the connecting cable or cables. These active electrical circuits include, for example, power distribution circuits, amplifiers, repeaters, multiplexers, and other signal conditioning and interpreting circuits. A result with a conventional electrical hydrophone array is that the hydrophone array itself, as well as the cable(s) connecting the array to the dry portion of the acoustical sensor, has to include a multitude of electrical conductors, is larger and much heavier than is convenient, and is expensive.
A particular disadvantage of these conventional electrical hydrophone arrays has been the presence of active electrical circuits within the array and connecting cable(s). These active electrical circuits require electrical power, thus requiring power distribution conductors and amplifiers in the wet portion of the array. The power distribution circuits and amplifiers utilize high voltages. Accordingly, it has followed inexorably that conventional electrical hydrophone arrays present problems with water leaking into the power distribution circuits and active electrical circuits of the array, possibly causing degraded performance because of increased capacitive coupling within the array, and also possibly causing electrical shorting in the array.
For personnel handling such conventional electrical arrays a potential shock hazard is also always present, and influences handling practices with such arrays. That is, even when the hydrophone array is really turned off, personnel have to treat it as though it were on and as though a shock hazard continuously existed. This precaution is necessary in order to establish and maintain safe handling practices.
Further, and undesirably, conventional electrical hydrophone arrays have involved a considerable expense to fabricate the wet portion of the array. This was the case because of the large number of electrical conductors in the array, the presence of the active electrical circuits, and the necessary attempts (frequently unsuccessful) both to make the array water tight during its lifetime, as well as to also be resistant to electrical malfunction, degraded performance, and shorting in the event that water leakage into the array did occur at some time during its useful life.
These problems with conventional electrical hydrophone arrays have led to the development of fiber-optic hydrophone arrays. These conventional fiber-optic hydrophone arrays use fiber-optic acoustical transducers. Light energy is conducted to and from the fiber-optic acoustical sensors along optical fibers extending in the cable portion of the array. No electrical wires are used in the connecting cable or in the array as in electrical hydrophone arrays. Accordingly, such fiber-optic hydrophone arrays do not include active electrical circuits or power distribution circuits in the wet portion of the array.
FIG. 12 illustrates a conventional architecture for a time division multiplexed (TDM) fiber optic hydrophone array. This architecture is conventionally referred to as a ladder network. In this figure, both the upper and lower lines represent fiber optic conductors. The rungs of the ladder are formed by hydrophones in the form of Mach-Zehnder interferometers (or possibly by Michelson interferometers) responsive to ambient acoustic energy. Between adjacent rungs of the ladder, a coil of the optical fiber provides a light propagation delay element having a period xe2x80x9cTxe2x80x9d. At the distal end of the upper conductor, a light pulse of duration xcfx84 less than T is applied. As this pulse proceeds to the right along the upper conductor, a coupler at each rung diverts a portion of the light energy of the pulse into the interferometer. The interferometer provides to the lower conductor, a pulse of light which is phase discriminated as a function of the ambient acoustic energy. As is illustrated, because of the time delay effected progressively along the length of the upper conductor, the pulses delivered into the lower conductor and in a distal (left) end of this lower conductor are time-division multiplexed relative to one another. In other words, the user of such an architecture can distinguish the signals from each particular one of the successive hydrophones along the length of the ladder array because of the arrival of the light pulse from each hydrophone in the train of pulses returned from the array in response to each input pulse. See, xe2x80x9cFiberoptic Sensorsxe2x80x94an introduction for Engineers and Scientistsxe2x80x9d, edited by Eric Udd, John Wiley and Sons, 1990, chapters 10 and 11. Also see, xe2x80x9cFiber-optic Sensorsxe2x80x9d, by T. A. Krohn, Instrument Society of America.
FIG. 13 illustrates another conventional architecture for a frequency division multiplexed (FDM) fiber optic hydrophone array. This architecture is conventionally referred to as a matrix network or topology. In this Figure, two continuous-wave light sources are provided, each modulated at a different frequency. Each light source provides light energy into a sensor, each sensor having a pair of interferometers responsive to ambient acoustic energy. The interferometers provide phase discriminated output signals to two output optical fibers. Viewing FIG. 13, the upper fiber carries two output signals, one from one interferometer of the upper pair and the other from one interferometer of the lower pair. The same is true of the lower output conductor. The signals on each conductor are distinguishable from one another because of their modulation carrier frequency. Thus, this arrangement is one of frequency division multiplexing, or FDM.
Those ordinarily skilled in the pertinent arts will recognize that this FDM matrix array concept can be expanded to an array having more than the four hydrophones seen in FIG. 13. That is, the matrix array may have N light sources each with its own modulation frequency different than the others, and N2 sensors; with N output conductors each carrying N signals distinguishable from one another by their modulation frequency. FIG. 14 illustrates a generalized FDM matrix array topology for a fiber optic acoustical sensor according to this concept.
Unfortunately, a persistent problem with fiber-optic hydrophone arrays has been the existence of xe2x80x9cstrumxe2x80x9d, or low frequency noise in the output signal from the array. This low-frequency xe2x80x9cstrumxe2x80x9d noise has severely impacted the performance of conventional fiber-optic acoustical sensors in the frequency range from less than one Hz to several hundred Hz. In fiber-optic acoustical sensors which include acoustical transducers connected to a light source and to a detector by a length of fiber optic cable, physical manipulation of this cable has a dramatic effect on the output signal noise. For example, in a towed hydrophone array in which a tow cable includes a single-mode optical fiber along which light from a laser light source is transmitted to a fiber-optic acoustical transducer, twisting, bending, and stretching of the tow cable in its noisy environment has a dramatic effect on the polarization state of the light in the optical fiber. In other words, the fiber-optic tow cable itself is in some respect a hydrophone exposed to a noisy environment including low frequency twisting, stretching, and bending occurring within the frequency band of interest. This tow-cable-hydrophone effect in turn affects the level of noise in the optical signal at the output of the acoustical transducer, which is transmitted along a return optical fiber to an optical receiver of the hydrophone array.
Further to the above, several conventional polarizers are well known. One polarizer of a fiber-optic type which is known is generally referred to as xe2x80x9cZingxe2x80x9d fiber. This particular type of optical fiber has a non-circular cross sectional configuration which preferentially propagates a selected polarization of light. Another type of polarizer is the so called xe2x80x9cbulkxe2x80x9d polarizer. A bulk polarizer uses a body of material (usually a crystalline substance) preferentially propagating a particular polarization of light. With some transparent crystalline materials, their regular crystal lattice structure makes them usable as bulk polarizers. Still another type of polarizer is known as a Brewster stack of plates. This so-called Brewster stack uses one or more plates made of a material having an index of refraction differing from air, and defining air-material interfaces at which polarizing reflections and refractions take place according to well understood principles of optics. With a Brewster stack of plates made of glass (index about 1.5), "THgr"B, the Brewster angle for polarizing incidence is defined as: "THgr"B=arctan nH/nL, in which nH and nL are the index of refraction of the material and of air (index 1). "THgr"B for glass plates is about 55xc2x0 to 57xc2x0. xe2x80x9cpxe2x80x9d polarized light is transmitted at the end interface without loss, whereas xe2x80x9csxe2x80x9d polarized light is largely reflected.
Unfortunately, not one of these conventional polarizers has a character which makes it inexpensive, rugged, small in size, conveniently used with optical fiber connection both on the light supply and polarized light delivery side of the polarizer, and offers very good extinction of an undesired polarization state in the polarized light delivered from such a polarizer.
In view of the above, a primary object for this invention is to avoid -one or more of the shortcomings of the related technology.
More particularly, it is an object for this invention to provide a fiber-optic acoustical sensor system in which the polarization state of light supplied to a fiber-optic acoustical transducer is controlled such as to reduce the effect of xe2x80x9cstrumxe2x80x9d of an elongate connecting cable between a light source and the acoustical sensor.
According to an exemplary embodiment of the invention, the hydrophone apparatus is configured as a static or towed array including such an acoustical sensor system having a xe2x80x9cdryxe2x80x9d portion on shore or aboard a transport vessel, and a xe2x80x9cwetxe2x80x9d portion immersed in the sea and including an elongate fiber-optic cable extending from the dry portion to an acoustic transducer of the wet portion. The wet portion of the static or towed array includes only comparatively inexpensive components (i.e., in the portion of the array which is immersed in the sea and which incurs a significant risk of loss or damage during use). The wet portion of such a fiber-optic towed array is additionally smaller in size and lighter in weight than a comparable hydrophone array of conventional electrical construction.
Thus, according to an aspect of the invention, a fiber-optic acoustical sensor system includes an elongate fiber-optic conductor having a pair of opposite ends; means for introducing a pair of orthogonally-polarized light beams into one of the pair of opposite ends; a polarizer receiving the pair of orthogonally-polarized light beams from a second of the pair of opposite ends, and providing an output light beam of substantially stable polarization; and an optical acoustical transducer receiving the output light beam of substantially stable polarization and also receiving ambient acoustical energy, the transducer responsively providing a modulated light beam signal analogous to a characteristic of the acoustical energy.
Accordingly, a towed hydrophone array embodying the present invention may be smaller and lighter than conventional electrical arrays, and will require less storage space aboard a transport vessel. Further, the use of this new hydrophone array will be easier because its lighter weight makes it easier to deploy into the sea and to recover. Also, there are no active electrical circuits in the wet portion of the array to present a hazard of electrical shorting or shock. Accordingly, personnel handling such an acoustical array need not observe xe2x80x9cshock hazardxe2x80x9d handling procedures.
In order to control the polarization state of light supplied to an acoustical transducer of the acoustical sensor system, a polarizer is interposed between a light source and the transducer. Preferably, this polarizer is configured as a fiber-optic in-line device, which is disposed immediately before the acoustical transducer with respect to light transmittal from the source to this transducer.
A particularly preferred embodiment of an in-line fiber-optic polarizer includes a pair of generally coaxially aligned optical fibers having oppositely angulated end surfaces confronting one another along an optical axis. These end surfaces are disposed on opposite sides of a multi-layer dielectric stack disposed at the Brewster angle relative to the axis of the fibers, and having alternating layers of differing refractive index. This multi-layer dielectric stack serves to polarize light passing from one of the pair of aligned fibers to the other through the dielectric stack.
In one embodiment, the pair of aligned fibers at their opposite ends each may have optical surfaces perpendicular to their axis. Each of the aligned fibers is held in a respective ferrule, so that grinding, polishing and other manufacturing operations can be conducted at comparatively low unit costs and with good precision. In order to couple light into and from the pair of aligned optical fibers, respective opposite connecting optical fibers each may have perpendicular end surfaces mating with the pair of aligned fibers at the respective opposite ends. One of these connecting optical fibers receives light from a source, such as a laser, and the other conducts polarized light from the polarizer to a fiber-optic acoustical transducer. An alternative embodiment of the polarizer includes a pair of ferrules which are xe2x80x9cpig tailedxe2x80x9d for ease of connection, and which sandwich between them a dielectric stack disposed substantially at the Brewster angle. Yet another embodiment of the polarizer provides a dielectric stack on a carrier member disposed between ends of optical fibers, and the carrier member is itself an element of the polarizer, and is transparent to light.
Accordingly, an additional object for this invention is to provide an in-line all fiber-optic polarizer. Yet another object for this invention is to provide such an in-line all fiber-optic polarizer as part of an apparatus for under water acoustical sensing.
Thus, according to yet another aspect, the present invention may be seen to provide an in-line fiber-optic polarizer including: a pair of optical fiber portions aligning along an optical axis and each defining a respective one of a confronting pair of end surfaces angulated with respect to the optical axis at substantially a Brewster angle; and a dielectric layer disposed between the pair of end surfaces, the dielectric layer including at least three sub-layers, including at least a pair of high-index sub-layers each adjacent to a respective one of the pair of end surfaces, and at least one low-index sub-layer disposed between the pair of high-index sub-layers, with each of the high-index and low-index sub-layers having a respective index of refraction determining the Brewster angle.
An advantage of the present invention results from the reduction in low-frequency noise component of the optical return signal obtained from such a fiber-optic acoustical sensor system, and resulting improved signal-to-noise ratio, which may be achieved in a fiber-optic acoustical sensor system by use of this invention. Actual tests of an embodiment of this invention have shown a 25 dB noise reduction in the 0 to 100 Hz. frequency range.
An additional advantage of the present invention results from the good polarizing performance, small size, relative structural simplicity, and commensurate low manufacturing cost of a fiber-optic in-line polarizer as described above. Because each acoustical transducer of an array of hydrophones may require one of the polarizers, the comparatively low cost for the polarizers reduces the overall cost of the array. Also, it is believed that a single polarizer may be used to service plural hydrophones. That is, the hydrophone array may be constructed in a branched configuration, with light from one polarizer dividing and branching out to several hydrophones. This branched construction would further reduce costs for the hydrophone array. Further, the good polarizing performance of the in-line polarizers contributes to desirable performance levels for the acoustical hydrophone array, while the small size of the polarizers helps allow a small, light, and easily handled array.
A better understanding of the present invention will be obtained from reading the following description of preferred exemplary embodiments of the present invention, taken in conjunction with the appended drawing Figures, in which the same features (or features analogous in structure or function) are indicated with the same reference numeral throughout the several views. It will be understood that the appended drawing Figures and description here following relate only to one or more exemplary preferred embodiments of the invention, and as such, are not to be taken as implying a limitation on the invention. No such limitation on the invention is implied by such reference, and none is to be inferred.